In the production of copper clad laminates for use in the manufacture of printed circuit boards it is not uncommon that, during the laminating process, the outer copper surface of the finished laminate is damaged by the entrapment of particulate matter between it and a pressure applicator. Pressure can be applied by a roller in continuous laminating or a hardened steel plate in press laminating.
The contamination can arise from resin particles used in the bonding process or dust particles lying on the copper at the time pressure is applied. The present of such particles, which are usually of microscopic dimensions, causes either indents on the surface of the copper layer or spots of resin melted onto the surface.
This contamination can interfere with the efficient production of circuit boards and is viewed with concern by the industry as it can cause rejects of the laminate before use and/or substantial pre-use surface preparation before the laminate can be processed into a printed circuit. Such surface preparation can be by brushing and pre-etching of the copper layer before processing.
The effect of this form of surface contamination of the copper layer is further complicated by the presence on the copper itself of an anti-oxidant layer put there during its manufacture. Such a layer is described in U.S. Pat. No. 3,853,716, which proposes an electrolytically deposited layer of chrome. Such layers are applied by the copper foil manufacturers so that during storage and laminating the copper layer is prevented from oxidizing through exposure to the atmosphere or during the laminating process where it is subjected to heat.
In order to be effective against oxidation such layers need to be of significant thickness and the consequence of this, plus the other contamination referred to above, is that the surface preparation of the laminate required before printed circuit board production is vigorous and potentially harmful to the laminate. This is particularly the case in so-called thin laminates (0.05 to 0.75 mm thick) where the risk of creasing the material is high. Where such thin laminate is also clad with thin copper (12-20 .mu.m thick) the damage risk is even greater.